If you have
ssh-askpass installed (or a variant like
seahorse-ssh-askpass), you can force a prompt by adding your key to the keyring with
ssh-add. Note that to get a graphical/UI prompt, you need to disable standard input, because
ssh-add will read input from the connected terminal if it is available.
# will use SSH_ASKPASS, or ssh-askpass by default
ssh-add ~/.ssh/id_rsa </dev/null
# use seahorse's prompt utility
SSH_ASKPASS=/usr/lib/seahorse/seahorse-ssh-askpass ssh-add ~/.ssh/id_rsa </dev/null
seahorse itself provides a UI for managing secrets backed by
gnome-keyring-daemon, but the main program is not meant for a one-shot prompt-and-save operation.